ADMIRAL OF THE FLEET LORD BOYCE (Martlet 1956 – 1961), who has died aged 79, was the Chief of the Defence Staff from 2001 to 2003 and challenged the Blair government's legal authority in the run-up to the Iraq War.

Michael Cecil Boyce was born in Cape Town on April 2 1943, the son of Commander Hugh Boyce and an Afrikaner mother, Madeleine, née Manley. His father recalled the prophetic words of the doctor who announced the birth of his first-born: "This little admiral is doing well!"

Mike inherited his faith, his self-restraint and his work ethic from his parents who, with their free church backgrounds, were firm believers in self-improvement, and skimped themselves to achieve good educations for their three boys: Philip Boyce became a professor of psychiatry in Sydney, and (Sir) Graham Boyce a diplomat. Mike was educated at Fernden School, Haslemere, and Hurstpierpoint College before joining Dartmouth in 1961 under the short-lived Murray Scheme, the last time the Navy attempted to give its officers a complete education.

During his fourth year under the Murray scheme, Boyce spent two weeks in the submarine Auriga and then, in 1965, he volunteered for "the Trade" (as the Submarine Service was nicknamed), and also specialised as a torpedo and anti-submarine warfare specialist. In 1970 he served briefly in the diesel-engined submarine Oracle, commanded by the future Admiral Sir Hugo White.

He was fortunate to be selected early to attend the "perisher", the make-or break course for potential submarine commanders and became, as a lieutenant-commander, commander of Opossum (1974-75)

In 1977-79 he was the first appointee as Staff Warfare Officer (Tactical Systems), responsible for introducing into the submarine service a new tactical data-handling system. As a newly promoted commander, he was given the nuclear-powered attack submarine Superb (1979-81) and was appointed OBE for special operations against the Soviet navy.

He rose swiftly in general service and was promoted to captain in 1982, commanding the frigate Brilliant (1983-84). He was a calm and understanding captain in charge of submarine sea training 1984-86, a leading student at the RCDS in 1988, and a successful Senior Naval Officer, Middle East, in 1989.

Subsequently, when his first marriage was breaking up, Boyce was Director of Naval Staff Duties (1989-91). Promoted to rear-admiral, he was Flag Officer Sea Training (1991-92), and as a vice-admiral Flag Officer Surface Flotilla (1992-95), and as a full admiral, he was Second Sea Lord (1995-97).

In 1997-98, Boyce was Commander in-Chief Fleet, Commander-in-Chief Eastern Atlantic Area and Commander Naval Forces North Western Europe, taking up these offices shortly after Tony Blair had become prime minister. For a few weeks in early 1998 Boyce also became acting First Sea Lord while his predecessor Sir Jock Slater was ill, and he formally became First Sea Lord in late 1998. He succeeded General Sir Charles Guthrie as Chief of the Defence Staff in February 2001.

During the 2001 invasion of Afghanistan he was ridiculed by the US defence secretary Donald Rumsfeld for contradicting Rumsfeld's view that the war would be over by the following year (it lasted 20 years); he also spoke out against America's proposed ballistic missile defence shield; he thought that the so-called war on terror was a distraction from the long-term threats provided by Russia and China; and he warned that participants in the Iraq War of 2003 would not be greeted as "liberators with flowers on the end of rifles".

Of the Iraq War, Boyce later told the Chilcot Inquiry that the government was without any cohesion in the way it approached the 2003 invasion.

Created a life peer on retirement in 2003, he sat as a crossbencher, speaking out about the risk to service personnel facing liability for their actions for which he claimed politicians are ultimately responsible. He gave evidence to the Iraq inquiry, and, though a member of the Top Level Group of UK Parliamentarians for Multilateral Nuclear Disarmament and Non-proliferation, was a strong advocate that the UK, as one of the P5, should retain a nuclear deterrent. He also launched scathing attacks on the government for Britain's "anorexic" fleet of warships.

Boyce's appointment in 2004 as Lord Warden of the Cinque Ports was inevitably greeted by the headline "Admiral sails into Cinque Ports". He was made Knight Companion of the Order of the Garter in 2011 and promoted to Admiral of the Fleet in 2014.

He was involved in some 60 charities and voluntary associations, including Chairmanship of the RNLI. He was most keen in his support of ADVANCE, the Armed Services Trauma Rehabilitation Outcome Study, which investigated the long-term physical and psycho-social outcomes of battlefield casualties. 

In 1971 Boyce married Harriette Gail Fletcher; they separated in 1994 and divorced in 2005, and in 2006 he married the South African widow of Vice-Admiral Malcolm Rutherford, Fleur (née Smith), who was his companion while he was First Sea Lord and Chief of the Defence Staff.

Fleur died in 2016 and Mike Boyce is survived by a son and a daughter of the first marriage and a stepson and stepdaughter.

Admiral of the Fleet Lord Boyce, born April 2 1943, died November 6 2022